1. Field of the Invention
The present invention relates to an optical writing type liquid crystal light valve which is used for a large-screen high-definition projection display and the like, a method of driving the same and a writing apparatus used for the same.
2. Description of the Related Art
FIG. 1 is a sectional view of a conventional light valve which is described in, for example, CONFERENCE RECORD OF 1990 INTERNATIONAL TOPICAL MEETING ON OPTICAL COMPUTING (1990), 9B3, pp. 17 to 18.
In FIG. 1, the reference numeral 1 represents a glass substrate, 2 a transparent conductive film composed of ITO (indium tin oxide) or the like, 4 a liquid crystal layer, 5 an orientation film for orientating a liquid crystal, 6 a reading light reflecting plate, and 7 a photoconductive layer composed of a hydrogenated amorphous silicon film which has a high electrical resistance.
The operation of this light valve will now be described. When data is written into a liquid crystal display device, a predetermined bias is first applied between an opposing pair of transparent conductive films. In this state, writing light having any given bright and dark space pattern is caused to enter the photoconductive layer 7. At this time, the photoconductive layer 7 has a high resistance at the points which are not irradiated with the writing light, while the resistance of the photoconductive layer 7 is lowered at the points which are irradiated with the writing light, due to the photoconductive effect. In this way, the carrier distribution which corresponds to the space pattern of the writing light is formed on the interface between the photoconductive layer 7 and the liquid crystal layer 4. In correspondence with the carrier distribution, a spatial distribution is produced due to the birefringence or the optical rotational power of the liquid crystal. For example, if a ferroelectric liquid crystal is used for the liquid crystal layer 4, the spatial distribution due to the birefringence of the liquid crystal layer 4 which corresponds to the space pattern of the incident light is produced.
The thus-written spatial distribution caused by the birefringence or the optical rotational power of the liquid crystal 4 is read by causing reading light as polarized rays to enter the liquid crystal layer 4 from the right-hand side in FIG. 1 through a polarizer and projecting the light reflected by the reading light reflecting plate 6 onto a screen through the polarizer.
FIG. 2 shows the structure of a liquid crystal display device as a projection display. The reference numeral 8 represents a liquid crystal light valve and 9 a voltage applying means. As the light writing means, a laser light source such as a CRT, or an He-Ne laser or a semiconductor laser combined with a two-dimensional scanner 10 is used. The reference numeral 11 denotes a reading light source, 12 a polarization beam splitter, and 13 a screen.
The laser light is scanned by the two-dimensional scanner 10 and an image is written into the liquid crystal valve 8. The liquid crystal valve 8 is irradiated with the light from the reading light source 11 which is polarized through the polarization splitter 12 as predetermined polarized rays. Since the liquid crystal valve 8 reflects the polarized rays respectively which are based on the written data, the polarized rays having a two-dimensional distribution which is determined by the written data are separated by the polarization beam splitter 12, and the image is displayed on the screen 13.
In this liquid crystal display device, the writing light and the reading light are emitted from different light sources. It is therefore possible to change the wavelength of a two-dimensional image or change incoherent light into coherent light by using this liquid crystal display device. Since the liquid crystal display device has a higher spatial resolution in principle than a light valve using BSO (Bismuth Silicon Oxide), it is possible to incorporate the liquid crystal display device into a large-screen high-definition projection system.
A liquid crystal having bistability, which is a component of a liquid crystal light valve, will now be described briefly.
As a liquid crystal which exhibits bistability, a ferroelectric liquid crystal is conventionally used. The operation of a ferroelectric liquid crystal will be described in the following.
In FIGS. 3 and 4, the reference numeral 14 represents a ferroelectric liquid crystal molecule. A long and narrow liquid crystal molecule such as that shown in FIGS. 3 and 4 exhibits an anisotropic reflective index between the major axial direction and the minor axial direction. The liquid crystal has spontaneous polarization 15, 16 in the vertical direction with respect to the major axis of the molecule in correspondence with the externally applied electric field. Even after the electric field is cut, the orientation of the liquid crystal remains stable. When this liquid crystal is used for a display device, the major axes of the liquid crystal molecules are oriented approximately parallel to the substrate. As shown in FIG. 4, the liquid crystal molecules are oriented in a first orientation stable state with respect to an electric field E15 applied in the direction of the reverse side of the drawing, and in a second orientation stable state 16 with respect to an electric field applied in the opposite direction to the reverse side of the drawing.
The change in the orientation due to an electric field is caused only when the energy applied from outside the liquid crystal, namely, the product of the electric field intensity (voltage) and the voltage applying time exceeds a constant critical value (the threshold property). The critical value of the product of the electric field intensity (voltage) and the voltage applying time is called a critical pulse area and hereinunder will be referred to as "CPA". When the orientation state of the liquid crystal is the first orientation stable state, even if the electric field E16 which does not exceed the CPA is applied to the liquid crystal in the direction opposite to the reverse side of the drawing during a certain voltage applying time, the orientation state of the liquid crystal does not change. If the applied voltage exceeds the CPA (this voltage is defined as a threshold voltage (Vth)), however, the orientation stable state of the liquid crystal is inverted.
As described above, although the photoconductive layer 7 has a high resistance in the dark state (hereinunder referred to as "dark resistance"), when the photoconductive layer 7 is irradiated with writing light, the resistance of the photoconductive layer 7 at the portion which is irradiated with the writing light is lowered due to the photoconductive effect, and the voltage applied to the liquid crystal exceeds the threshold (Vth), and the orientation state of the liquid crystal changes. This operation is shown in time series in FIGS. 5a to 5c. The time axes are common to FIGS. 5a to 5c. In FIG. 5a, the symbol Vappl represents the waveform of the voltage applied between the transparent conductive films 2, A a reset pulse and B a writing period. The reference numeral 21 represents a writing voltage pulse, and 22 and 23 erasing light and writing light, respectively, projected onto the photoconductive layer. In FIG. 5b, the symbol V.sub.LC represents a voltage applied to the liquid crystal layer, and in FIG. 5c, the symbol Irrl represents the intensity of the reflected reading light at the portion which is irradiated with light.
The space between the opposing electrodes (transparent electrodes) 2 is first irradiated with erasing light in synchronism with a reset pulse for applying -Vth so that the entire liquid crystal layer assumes one orientation stable state. The liquid crystal layer is then irradiated with writing light having a predetermined two-dimensional pattern in synchronism with the writing period. Since a voltage exceeding the threshold Vth is applied to the liquid crystal layer at the portion which is irradiated with the light due to the photoconductive effect, the orientation state changes and the intensity of reading light is accordingly distributed.
By using a laser light source combined with the two-dimensional scanner 10 shown in FIG. 2 as the writing light projecting means, it is possible to record any given two-dimensional image on a liquid crystal light valve. The thus-written image is obtained by projecting the reading light with which the liquid crystal layer is irradiated through a polarization beam splitter 12 and which is separated by the polarization beam splitter 12, optically rotated in the liquid crystal layer 4, and reflected by the reading light reflecting plate 6 onto the screen 13. The optical rotation by the liquid crystal reaches its maximum when the angle 2.theta. between the first orientation stable state and the second orientation stable state is 45.degree..
In order to realize the writing operation by the liquid crystal display device, it is necessary that the resistance of the photoconductive layer 7 which is not irradiated with writing light is sufficiently higher than that of the liquid crystal layer 4, while the resistance of the photoconductive layer 7 which is irradiated with writing light is not more than the resistance of the liquid crystal layer 4. In order to realize the high spatial resolution of the device, it is necessary that the film thicknesses of the liquid crystal layer 4 and the photoconductive layer are small in addition to the condition that the dark resistance of the photoconductive layer is sufficiently high.
In the above-described conventional liquid crystal display device, a hydrogenated amorphous silicon film is used for the photoconductive layer 7, or CdS, crystalline silicon and BSO are conventionally used. Among these, a hydrogenated amorphous silicon film, which is formed by plasma CVD, is advantageous in that it is comparatively easier to set a resistance value in comparison with any other material, in that the adhesiveness with an underlayer is high and in that the photosensitivity in the visible light range is high.
However, the dark resistance of the hydrogenated amorphous silicon film formed by plasma CVD (Chemical Vapor Deposition) is 1.times.10.sup.9 to 1.times.10.sup.11 .OMEGA..cm in the case of a non-doped film, and at most 1.times.10.sup.12 .OMEGA..cm in the case of what is called a boron lightly doped film. These resistances cannot be said to be sufficiently higher than the general resistivity 1.times.10.sup.11 to a.times.10.sup.12 .OMEGA..cm of a liquid crystal. It is therefore necessary to increase the dark resistance of the hydrogenated amorphous silicon film by some means in the case of using it for the photoconductive layer 7.
For this purpose, a method of making the hydrogenated amorphous silicon film much thicker than the liquid crystal layer 4 (e.g., the hydrogenated amorphous silicon film is formed to a thickness of about 10 .mu.m while the liquid crystal layer 4 is 2 .mu.m thick), and a method of forming the photoconductive layer 7 from a hydrogenated amorphous silicon film having a pin structure and executing the writing operation by applying a reverse bias to the pin photodiode are conventionally adopted. However, if the thickness of the photoconductive film 7 is increased, the spatial resolution of the liquid crystal display device is disadvantageously lowered. In addition, if the photoconductive layer 7 has a pin structure, the waveform of the voltage applied to the liquid crystal layer 4 becomes different from the waveform of the driving voltage, so that the liquid crystal deteriorates during periods of long driving.
The operation temperature of the liquid crystal display device is limited to a definite temperature range. If this temperature range is higher than room temperature, the dark resistance of the hydrogenated amorphous silicon film and the activating energy of the reverse current of the pin photodiode are higher than those of a liquid crystal, and it is thus difficult to make the dark resistance of the photoconductive layer 7 higher than the resistance of the liquid crystal layer 4.
In the above-described conventional liquid crystal display device, if a CRT is used as an optical writing means, the resolution of the optical writing type liquid crystal light valve is limited by the resolution of the CRT as the light projecting means. On the other hand, if a laser light source combined with the two-dimensional scanner shown in FIG. 2 is used as the optical writing means, it is possible to increase the spatial information density of the writing light by converging the laser beam onto the photoconductive layer and using this beam for raster scanning, so that the optical information recorded with a high resolution which is characteristic of an optical writing type liquid crystal light valve can be expected.
As described above, the dark resistance of the photoconductive layer 7 cannot be said to be sufficiently higher than the resistance of the liquid crystal layer 4.
Therefore, if a writing voltage is applied between the transparent conductive films 2 during the writing period B, a distributed voltage which is determined by the resistance between the photoconductive layer 7 and the liquid crystal layer 4 is applied to the liquid crystal layer 4. Therefore, if the writing period B is long as in the case of conducting optical writing with high definition by a laser light source combined with the two-dimensional scanner 10, the energy applied to the liquid crystal layer 4 which is not irradiated with light exceeds the CPA, which changes the orientation in the area which is not irradiated with light and the intensity of the reflected light at this portion gradually rises disadvantageously, as indicated by the broken line C in FIG. 5c.
FIG. 6 is a timing chart for explaining the procedure for writing data into a liquid crystal light valve which is applied to a conventional image-forming device described in Japanese Patent Laid-Open No. Hei 1-241528. In this example, a laser beam is converged onto a liquid crystal light valve in order to scan, and an image is written by utilizing the lowering of the resistance of the photoconductive layer 7. In FIG. 6, (a) represents timing for a timing signal generated by a sweep starting signal, (b) timing for a driving field for driving a liquid crystal light valve and (c) timing for a video signal for modulating the laser light.
The operation of the conventional liquid crystal display device will be explained. In writing data into the liquid crystal light valve, raster scanning is conducted by a laser beam through a polygon mirror scanner and a galvanometer mirror scanner, and a square wave having both positive and negative polarities being applied during scanning, as indicated by (b).
The writing operation is carried out at a specific polarity of the driving voltage. In this example, the video signal indicated by (c) is supplied when the polarity is negative and the writing operation is conducted. A positive voltage is applied so as to cancel the direct current component of the pulse during writing and while the voltage is applied, the writing operation is suspended.
In the above-described conventional method of writing data into the liquid crystal light valve, the writing operation must be suspended during the application of a voltage having an opposite polarity to that of the applied writing voltage.